Although exposure to asbestos has been definitely associated with the development of lung cancer, the magnitude of the risk has not been defined in persons who are exposed to relatively minute doses of asbestos. Even small amounts of asbestos together with cigarette smoking, have been shown to have a synergistic effect in producing lung cancer (1). In a preliminary study, we have found large numbers of asbestos bodies more frequently in our lung cancer patients than in a control population without lung cancer. This occurs in cases where asbestos bodies are easily overlooked on routine histologic sections. We now propose to confirm this finding by quantitating the number of asbestos fibers in the lungs of more patients with and without lung cancer in a population with low asbestos exposure. Quantitation of fibers will be performed at both light and electron microscopic levels, and specific identification of fibers as asbestos will be carried out by morphology, electron diffraction, and microprobe analysis. It is hoped that quantitative criteria of "significant" exposure to asbestos can be determined, and the magnitude of the problem of increasing evidence of asbestos in the environment clarified.